1. Field of the Invention
This invention is in the field of methods and means for the production of printed-wireboard circuits. More particularly, the present invention is in the field of improvements in the application of coatings to printed wireboards (PWB), sometimes also referred to by the term "circuit board".
2. Description of the Prior Art
Photoimageable solder masks are generally coatings capable of selective polymerization by the application of energy, such as, e.g., radiation. Such masks are applied as manufacturing aids in the process used to solder electronic components to printed wireboards, and comprise two basic types: those applied by screen-printing imaging techniques, and those applied by photoimaging techniques. The photoimageable solder mask is applied to the PWB, and irradiated; sites where radiation impinges thereafter become relatively more or less polymerized or hardened, depending on particular polymer characteristics, and thus more or less resistant to removal by washing. The sites where the polymer remains are therefore selectively protected from the adherence of solder when the PWB is dipped in a bath or otherwise treated to apply solder to the PWB surface. The more common method is that of negative imaging, where the area to be coated is defined as the clear portion of the image, and the area not to be coated is dark; irradiation through this mask or phototool then causes hardening of the polymer in areas of exposure. In either case of a relatively higher or lower degree of polymerization, an image is produced in conformity with a predetermined pattern, generally an electrical or electronic circuit.
In one process, solder mask is applied to the PWB in two stages, the first stage being a non-imaging screen-printing application, and the second a transfer-coating process.
In the transfer-coating process particularly, the solder-mask transfer layer is applied to a support layer via non-imaging screen-printing simultaneously with a first-stage coating applied to the PWB. This support layer, described further hereinbelow, is transparent, and includes the phototool, or photographic image of the desired printed circuit or portions thereof, used for imaging.
The transfer is completed by first mating the support and transfer layer to the PWB via roller lamination, and then exposing the resulting structure to ultraviolet (UV) radiation. With this exposure, as noted hereinabove, the solder mask is selectively polymerized such that it reproduces the image on the phototool element of the support layer. The imaged portions of the solder mask adhere to the PWB as the support layer is removed.
The support layer includes a 0.013-millimeter (mm) thick poly(ethylene terephthalate) or equivalent dimensionally-stable film, laminated with a 0.013 mm adhesive layer to a standard 0.10 or 0.18 mm diazo phototool. While the film is flexible, it is virtually rigid in length and width, and necessarily not easily deformed by heat or mechanical action, in order to insure accuracy in the imaging process. Unfortunately, this rigidity does not allow the support layer to conform to the irregular topography of the PWB surface. The resulting coating, in areas where it is thereafter further treated to produce an image on the PWB or selectively to remove the solder mask, can then interfere with steps such as soldering components to the PWB, or can be too thick from the perspective of economically further treating the board.